Carriker : List of the Birds of Costa Rica. 319 



difficulty, while some of the well known, wide-ranging species cannot 

 be classed as characteristic of any one zone, it even being very uncer- 

 tain whether they have originated from the Tropical or Boreal faunas. 



The geographical position and meteorological peculiarities of Costa 

 Rica make possible the presence of the enormous bird-fauna to be 

 found within its confines, at the same time greatly increase the diffi- 

 culty of a satisfactory disposition of many of the species with respect 

 to the life-zones. The continent has narrowed down from three thou- 

 sand miles in breadth to scarcely more than sixty at the narrowest part 

 of Costa Rica, while within that sixty miles are crowded a diversity 

 of climatic conditions, humidity, and altitudes scarcely to be paralleled 

 on the face of the globe. 



The northern and southern forms of the Pacific and Caribbean low- 

 lands here meet, and overlap ; an arm of the Sonoran occupies the 

 central plateau, a single remnant of the Canadian persists on the iso- 

 lated peaks of a few high mountains, while the Tropical forms contest 

 nearly all the territory with the Sonoran. With such a multitude of 

 species and with so many diverse conditions brought into such close 

 proximity, it is very evident that there must be a continual struggle 

 for ascendancy between the types of the various life-zones, that many 

 highly specialized forms only persist within their respective areas, 

 while others more plastic, have adapted themselves to a wide diversity 

 of environment and are to be found ranging over the whole of the 

 respective zones of which they are typical, as well as penetrating into 

 adjacent territory. Thus may be explained the persistent overlapping 

 of birds from one zone in another and the presence of the so-called 

 neutral belt lying between them. In defining these zones, these wide- 

 ranging species must be quite ignored, and only those considered the 

 ranges of which are limited to regions having some physical character- 

 istic not found beyond that immediate locality, such as temperature, 

 altitude, or humidity. 



The fact that certain zones extend to lower altitudes in some places 

 than at others is largely influenced by "slope exposure." Where the 

 slope of the mountain range is abrupt, from a point near sea-level, 

 without intervening foothills or table-land, we find the Cordilleran 

 forms descending to much lower altitudes than where such obstacles 

 intervene, these hills or table-lands, when present, always tending to 

 warm the slope to a higher altitude than would otherwise be the case. 

 Localities with, or without, these conditions can be compared only 



